This invention relates to a method for forming two complementary patterns during processing of a semiconductor material while using a single masking step, and, more particularly, to a method for doping semiconducting material with different types of dopants and/or different concentrations of the same dopant, such as to form N-type and P-type semiconductor regions, while using a single masking step.
In forming semiconductor integrated circuits, it may be necessary or desirable to form complementary patterns, or regions, such as N-type and P-type, disposed in a semiconducting material like silicon. Complementary patterns are used to define a first area of an object wherein it is desired to perform a first action, such as dopant implantation, on the first area while blocking a second area of the object from the first action and then performing a second action, such as dopant implantation, on the second area while blocking the first area from the second action.
A common method uses two masking steps. In the first step a portion of a semiconductor material is masked and a first dopant is implanted in the unmasked part of the semiconductor material. For the second step, the part of the semiconductor material having received the first dopant is masked and a portion of the semiconductor material that was previously masked is unmasked so that a second dopant can be implanted therein. For example, when the semiconductor material comprises silicon, the first dopant may be phosphorous or arsenic for providing an N-type region and the second dopant may be boron for providing a P-type region.
Two masking steps result in increased processing costs, increased process complexity (the second mask must be properly aligned and registered) and generally cause reduced yield of desired components over that achievable using one masking step. Therefore, it would be desirable to eliminate one masking step so that complementary patterns could be formed using a single masking step.
Another method, which uses a single masking step, employs a lift-off process. A first masking pattern of photo resist is established on the surface of the semiconductor material, which leaves predetermined portions of the surface of the semiconductor exposed. A first dopant is implanted into the semiconductor material through the portion of the exposed surface. A second masking pattern is formed by depositing metal over the photo resist of the first pattern and over the first implanted regions of the semiconductor. The photo resist along with the metal deposited on it is removed, leaving behind the metal overlaying the first implanted regions, which constitutes the second mask pattern. A second dopant is implanted through the parts of the surface of the semiconductor material that were previously covered by the photo resist of the first pattern and then the remaining metal is removed.
There are disadvantages associated with this lift-off technique. Metal deposition is typically performed in a high vacuum metal deposition system which requires subjecting the semiconductor material along with the photo resist of the first pattern to a high vacuum, such as less than about 10 m Torr. Photo resist as generally used for masking includes solvents that outgas when exposed to the vacuum, potentially contaminating the deposition process and rendering it harder to control. If the contamination is severe enough, the circuit may cease to function.
It would be desirable to provide a single masking step that could be used along with dopant implantation of semiconductor material that could accurately and reproduceably form complementary patterns in the semiconductor material. Further, it would be desirable to have a masking process that is easy to control and that is compatible with existing semiconductor processing technology.
Accordingly, it is an object of the present invention to provide a method for forming complementary patterns while using a single masking step, wherein the method is easy to control and is compatible with existing semiconductor processing technology.
Another object is to provide a method that can accurately and reproduceably form complementary patterns in a semiconductor material while using a single masking step.